In electrical energy distribution systems in urban areas, interconnection lines connect the nodes of a complex network constituted by transformer stations, in which medium to low voltage transformers supply energy to the network, and by junction or interconnection boxes, in which several lines are interconnected. The distribution system is three-phase and the distribution lines contain a plurality of cables for each phase. This is necessary in view of the high current transmitted along the lines.
The cables of the various phases forming a line are often impossible to distinguish and are distributed in a disorderly manner. Identifying the phase to which one of these cables belongs is consequently a difficult and dangerous operation. In fact, work often has to be carried out on lines while they are live.
Also in other situations, for example, when connecting equipment to a three-phase line, it may be necessary to identify the individual phases of an electrical power supply system in a fast and safe manner, even without disconnecting the power supply.
WO-A-2006/100695 describes a phase detecting system based on the use of a transmission device and a receiver device, applied in two points of the three-phase network, to cables that can belong to the same phase or to different phases. To identify the phase on which the receiver device is located, the transmission device sends a message on the electrical line to which it is connected, in phase with the voltage on said line. The receiver device recognizes the phase to which it is connected on the basis of the phase shift between the voltage waveform detected on the phase to which it is connected and the message received by the transmission device.
This system is particularly efficacious and useful, especially to rapidly identify the phases of a three-phase system in a complex electrical distribution network, but has the drawback that the transmission and receiver devices must be connected galvanically to the network cables. As it may be necessary to carry out various readings on different cables to identify a specific phase, this means that the insulation must be cut and then restored on a plurality of different cables for each operation, resulting in time loss, deterioration in the electrical insulation properties of cables and risk for operators taking the measurements.
US2004/263147 describes a phase detecting system based on a different concept. In this case the phases of a three-phase system are identified by detecting the phase shift between the voltage waveform and a time signal supplied by the GPS system. Phase shift between GPS signal and voltage waveform measured in two different points of an electrical distribution network allows identification of the phase. This system, particularly useful for very extensive networks, where the points in which the readings are carried out are at a distance even of several tens of kilometers from one another, has a substantial drawback arising from the fact that a GPS signal must be supplied in all points in which the phase of the system must be identified. Moreover, this system requires electrical contact with the network cable in order to detect and identify the phase.